%checks type refinement function for opaque alias types

[lll000111]

I'm using opaque alias types described here: https://flow.org/en/docs/types/opaque-types/

I'm also using the undocumented checks% hack to define type refinement functions, for example

declare function isHash(x: mixed): boolean %checks(typeof x === "string");

and the actual function is defined as

function isHash (thing: mixed): boolean {
    return typeof thing === 'string' && StorageBaseCommon.CRYPTO_HASH_RE.test(thing);
}

(I have a reason for using a separate declare statement to have the %checks on instead of putting it right on the actual function, but that's not important now.)

My problem is that I don't seem to have a way to write a type refinement function for an opaque alias type?

For example, I use SHA-256 hashes in my code, which are strings, but very special strings (obviously). So I'm using an alias

export opaque type SHA256Hash: string = string;

---

It's very unfortunate that I can't just tell Flow that a function checks a given type, that I can only use typeof and instanceof. That only works for "fake OOP" class-based programming where instanceof works. It doesn't work for a functional style when I have just different kinds of plain objects and no inheritance and nothing is constructed using new. And plain types cannot be aliased if only typeof is allowed for refinement.

[panagosg7]

There are some limitations with refinements on opaque types:

• Opaque types can be refined based on their type body in the same file. try-Flow
• Opaque types can not be refined based on the defined super-type. try-Flow

This issue is not really related to the %checks functionality, though the limitations apply there as well.